Much work has been done over the years to convert heavy petroleum feedstocks to lighter and more valuable liquid products. One process developed through the years for accomplishing this conversion is fluid coking. In conventional fluid coking, a heavy petroleum feedstock is injected into a fluidized bed of hot, fine coke particles and is thus distributed uniformly over the surface of the coke particles where it is cracked to vapors and coke. The vapors pass through a cyclone which removes most of the entrained coke particles. The vapor is then discharged into a scrubber where the remaining coke particles are removed and the products cooled to condense heavy liquids. The resulting slurry, which usually contains from about 1 to about 3 weight percent coke particles, is recycled to the coking reactor. The overhead products from the scrubber are sent to fractionation for separation into gas, naphtha, and light and heavy gas oils.
The coke particles in the reactor vessel flow downwardly to a stripping zone at the base of the reactor where stripping steam removes interstitial product vapors from, or between, the coke particles, as well as some adsorbed liquids from the coke particles. The coke particles then flow down a stand-pipe and into a riser which leads to a burner where sufficient air is injected for burning part of the coke and heating the remainder sufficiently to satisfy the heat requirements of the coking reactor where the unburned hot coke is recycled thereto. Net coke above that consumed in the burner is withdrawn as product coke.
Another type of fluid coking process employs three vessels: a reactor, a heater, and a gasifier. Coke produced in the reactor is withdrawn, and is passed through the heater where a portion of the volatile matter is removed. The coke is then passed to a gasifier where it reacts, at elevated temperatures, with air and steam to form a mixture of carbon monoxide, carbon dioxide, hydrogen, nitrogen, water vapor, and hydrogen sulfide. The gas produced in the gasifier is heat exchanged in the heater to provide part of the reactor heat requirement. The remainder of the heat is supplied by circulating coke between the gasifier and the heater.
Still another type of fluid coking process is a so-called once-through coking process wherein the bottoms fraction from the scrubber is passed directly to a hydrotreating unit instead of being more conventionally recycled to extinction. The disadvantage with such a once-through process is that the bottoms fraction is so laden with fine coke particles that plugging of the hydrotreating unit occurs.
Consequently, there exists a need in the art for a fluidized coking process which is not limited by the disadvantages of the prior art and which results in a scrubber bottoms fraction substantially free of solids.